Non-contact holder for wafer-like articles

ABSTRACT

A holder for wafer-like articles is formed by providing a platform having a wafer-like article facing surface that includes at least one annular groove therein. The annular groove has a ceiling surface therein which is provided with an opening. A gas conducting conduit is connected to the opening. A gas, which introduced into the conduit, exits the conduit through the opening. By choosing a particular orientation of the conduit relative to the annular groove, the gas exiting the opening can be caused to circulate in a clockwise or counter clockwise fashion. The circular flow of gas causes the formation of a vortex adjacent to the article facing surface. A wafer-like article may be held in a suspended state adjacent to the wafer-like article facing surface without contact by the vortex and the gas flowing between the wafer-like article and the article facing surface.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application Ser.No. 60/018,782, filed on May 31, 1996 and PCT International ApplicationPCT/US97/08626, filed May 21, 1997, wherein the United States is thedesignated country.

BACKGROUND

1. Field of the Invention

The present invention relates to a holder for holding wafer-likearticles without physical contact between the holder and the wafer-likearticle being held. More particularly, the present invention relates toan apparatus for holding articles through the use of a particular fluiddynamic relationship between the wafer-like article and the holdercreated by the structure of the holder.

2. Description of the Prior Art

The handling of wafer-like articles, such as silicon wafers, can presentproblems. Some prior art wafer-like article holding, apparatus havemechanical latches to hold the article in place. The mechanical latcheshave drawbacks such as contact between the article and the holder. Thecontact can cause contamination of the article as well as inducemechanical stresses in the article.

Other prior art wafer-like article holders include vacuum orelectrostatic chucks. These types of holders also have drawbacks. Suchdrawbacks include physical contact between the holder and the articlewhich can cause contamination and mechanical damage. Also, because someactual pre-processed articles, such as wafers, are not flat, securingsuch articles on a flat holder by vacuum or electrostatic means cancause mechanical stresses in the article.

U.S. Pat. No. 5,080,549 discloses a wafer handling apparatus operatingunder the Bernoulli principle to pick up, transport and deposit wafers.The apparatus includes a plate that has a plurality of laterallyoriented outlets and a central outlet for discharging gas in a patternsufficient to develop a low pressure environment to pick up the waferwhile bathing the wafer in radially outflowing gasses to preventintrusion and deposition of the wafer of particulate matter insuspension. The pressure differential caused by the Bernoulli principalserves to lift the wafer without any physical contact whatsoever betweena wand assembly and the top or bottom surfaces of the wafer.

SUMMARY OF THE INVENTION

The present invention provides a holder for holding wafer-like articleswithout physical contact between the holder and the wafer-like articlebeing held. The holder comprises a platform having an article facingsurface which includes at least one annular groove therein formed by aninner wall, an outer wall and a ceiling surface between the inner andouter walls. The ceiling surface is provided with an opening. The holderfurther includes a gas conducting conduit which is connected to theopening in the ceiling surface of the annular groove. The gas conductingconduit is oriented relative to the opening in the ceiling surface ofthe annular groove so as to cause a circular flow of gas around theannular groove when a gas is flowed down the conduit and through theopening.

One object of the present invention is to provide a wafer-like articleholder that avoids physical contact between the wafer-like article andthe holder.

Another object of the present invention is to provide a wafer-likearticle holder that does not induce mechanical stress in the wafer-likearticle while it is being held.

The foregoing and other objects, features, and advantages will becomeapparent from the detailed description of the preferred embodimentsinvention as illustrated in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings, which are not drawn to scale, include:

FIG. 1A, which is a diagram plan view of one embodiment of the apparatusof the present invention;

FIG. 1B, which is a diagram plan view of an alternative embodiment ofthe apparatus of the present invention;

FIG. 2A, which is a sectional view of the embodiment illustrated in FIG.1A taken along the line 2A--2A, and which also includes a wafer-likearticle adjacent to the apparatus;

FIG. 2B, which is a sectional view of the embodiment illustrated in FIG.1A taken along the line 2B--2B, and which also includes a wafer-likearticle adjacent to the apparatus;

FIG. 3, which is a graph showing the relationship of the differentialpressure with radial distance from the center of an annular groove inthe apparatus;

FIG. 4, which is a graph showing the relationship between holding forceapplied to the wafer-like article and the distance between the surfaceof the wafer-like article adjacent the apparatus and the apparatus;

FIG. 5, which is a diagram plan view of another embodiment of theapparatus of the present invention illustrating four annular grooves;

FIG. 6, which is a schematic diagram plan view of an end effector whichincorporates the structure of the present invention for creating avortex;

FIG. 7, which is a schematic diagram elevational view of the endeffector shown in FIG. 6; and

FIG. 8, which is a partial schematic diagram elevational viewillustrating a rim comprised of fingers for preventing excessive lateralmovement of the wafer- like article.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIGS. 1A and 2, the wafer-like article holder of thepresent invention comprises a platform 10 having a flat wafer-likearticle facing surface 11. The flat wafer-like article facing surface 11includes at least one annular 15 groove 12 therein formed by an innerwall 17 having a diameter d₁, an outer wall 18 having a diameter d and aceiling surface 14 between the inner 17 and outer 18 walls. The ceilingsurface 14 is provided with an opening 24. The wafer-like article holderof the present invention may also include a rim 13 about the perimeterof the platform 10.

Still referring to FIGS. 1A and 2, a gas conducting conduit 15 having adiameter m is provided in the platform 10 between the opening 24 in theceiling surface 14 of the annular groove 12 and a side 22 of theplatform 10 at opening 25. The conduit 15 is oriented in one aspect suchthat a component of its axis is substantially tangential to thecircumference of the annular groove 12 in the plane 25 formed by thearticle facing surface 11. Preferably, the ceiling surface 14 issubstantially parallel planar to the plane formed by flat wafer-likearticle facing surface 11. The conduit 15 is also oriented in anotheraspect such that a component of the axis of the conduit 15 makes anangle b which is generally less than 90°, and preferably less than 45°,relative to the parallel plane formed by the ceiling surface 14 ofannular groove 12. Also, preferably, the diameter m of the conduit 15 ismade smaller than the annular groove's depth H.

During operation, opening 25 is connected to a source of compressed gas.The compressed gas is flowed down conduit 15 to opening 24 where itexits. Because the conduit 15 is oriented relative to opening 24 theexiting gas flows around the annular groove 12. The gas flowing aroundthe annular groove 12 causes a vortex to be formed adjacent to theannular groove 12. As will be explained below, the gas dynamics providedby the vortex hold the wafer-like article in place without contact.

Alternatively, as shown in FIG. 1B, the ceiling surface 14 of theannular groove 12 may be provided with a plurality of openings 24a-24dand a plurality of conduits 15a-15d providing a flow of compressed gasto the openings 24a-24d. Each of the conduits 15a-15d is oriented in thesame manner as described above and in such a manner that each directs aflow of compressed gas out of the openings in the same direction aroundthe annular groove 12. The advantage to having a greater number ofopenings 24a-24d and conduits 15a-15d is that the vortex created can bebetter controlled so that it has less discontinuities.

The internal diameter d, of the annular groove 12, which is establishedby the inner wall 17, is chosen from the ratio system: ##EQU1## where:d--diameter of the outer wall 18 of the annular groove 12 which isestablished by the outer wall 18;

n--quantity of the openings 24 in the ceiling surface 14 of the annulargroove 12; and

S(m)--cross-sectional area of the opening 24.

Through adjustments in the orientation of the gas conducting conduit 15relative to opening 24 in the ceiling surface 14, as well as adjustmentsin the size of the annular groove, the gas dynamics created by theapparatus, i.e., the created vortex, can be optimized.

The wafer-like article holder of the present invention holds awafer-like article 16 adjacent the article facing surface 11 throughfluid dynamics caused by the delivery of compressed gas to opening 25.The compressed gas is directed via conduit 15 to the opening 24 in theceiling surface 14 of the annular groove 12. The gas exits the opening24 and moves around the annular groove 12 in either a clockwise orcounter clockwise direction, depending on the orientation of the conduit15 relative to the opening 24, and therefore, relative to annular groove12. In either case, the clockwise or counterclockwise flowing gas formsa vortex. At the center of the vortex there is a region of relativelylow pressure. As the angle b between the axis of the conduit 15 and theceiling surface 14 of the annular groove 12 is decreased towards 0degrees, the region of relatively low pressure caused by the vortexincreases.

The diameter m of the conduit 15 should be made smaller than the depth Hof the annular groove 12 to insure that the gas flows uniformly insidethe annular groove 12 to create the vortex. If the annular groove'sinternal diameter d₁, established by inner wall 17 is chosen accordingto the ratio system (1) expressed above, such that the annular groove'scross-section (d-d₁) established by the inner 17 and outer 18 walls, andcorrespondingly the ceiling surface 14 width, is larger than area ofopening 24, vortex gas velocity reduction due to annular groove-gasdynamical resistance will be minimized or avoided altogether.

By flowing gas through opening 24 according to the present invention, awafer-like article 16 may be held in place by merely positioning thewafer-like article 16 parallel to the flat wafer-like article facingsurface 11 with the gap of about 0.1-1 mm between the wafer-like article16 and the flat wafer-like article facing surface 11. The wafer-likearticle will be held in place adjacent to the flat wafer-like articlefacing surface 11 without physically contacting it.

The gas flowing out from the annular groove 12 flows along gap h betweenthe wafer-like article 16 and flat wafer-like article facing surface 11and forms radial flow which is continuous around the annular groove 12and directed out of it. As a result of the gas flow, a differencebetween the ambient gas pressure and the pressure in the gap h betweenthe wafer-like article 16 and flat wafer-like article facing surface 11is created. The typical pressure differential distribution as a functionof radial distance is shown in FIG. 3.

Referring to FIG. 3, within a region in the annular groove 12 where r isless than d/2, the ambient gas pressure P_(a) (e.g. atmosphericpressure) is higher than the gas pressure P_(h) in the gap h, and theresulting pressure difference ΔP (ΔP=P_(a) -P_(h)) is positive. This isthe reason why the wafer-like article 16 is pressed to the wafer-likearticle holder 11 with force: ##EQU2##

For a radius greater than the annular groove 12, d/2, the ambient gaspressure is lower than the gas pressure in the gap h, and the pressuredifference ΔP is negative. This is the reason why the wafer-like article16 is pushed out of the wafer-like article holder with force (see FIG.3): ##EQU3## This force F(-) does not allow the wafer-like article 16 totouch the wafer-like article facing surface 11.

Both force values and their sum F=F(-)+F(+) depend on the gap size h.The wafer-like article equilibrium location is a gap size h where thewafer-like article weight W is compensated by the force F. Twoconfigurations of the wafer-like article 16 with respect to the holder'ssurface 11 that meet the equilibrium conditions are when: (1) thewafer-like article 16 is placed below the holder surface 11: and (2) thewafer-like article 16 lies above the holder surface 11. For the firstcase, provided the wafer-like article's weight W is less than themaximum net positive force. F_(max) of FIG. 4, the wafer-like article isdrawn towards the holder surface 11 to an equilibrium position with gaphl. At this position, the downward force W is equal in magnitude andopposite in direction to the upward force supplied by the gas dynamicscreated by the holder. In the second case, gravity pulls the wafer-likearticle toward the holder surface 11 to an equilibrium position with gaph2 less than gap hi where the negative force, pushing the wafer-likearticle 16 away from the holder surface 11, balances the wafer-likearticle's weight W. Similarly, for any orientation of the wafer holdersurface to the direction of gravity, there is an equilibrium gap h whichthe component of W normal to the holder surface 11 is balanced by acounteracting force F supplied by the holder 11. Consequently, providedthere are retaining elements 13 (FIG. 2B & 8), the holder can hold thewafer-like article 16 in any orientation with no contact of thewafer-like article to the holder surface.

Analyzing FIG. 4, the advantages of the wafer-like article holder of thepresent invention become very apparent for the case when the holder isused to suspend the wafer-like article 16 because: a) the wafer-likearticle equilibrium position h1 is a stable position because of smalldeviation of the wafer-like article's position causes counter forces toarise, that is important for reliable wafer-like article holding; b) theholder may "catch" the wafer-like article and bring it to it'sequilibrium position h1 from any distance in the range between h1 and h3where the attractive force is higher than wafer-like article's weight,which may simplify loading the wafer-like article onto the holder; andc) due to the gas gap between the wafer-like article and the holder thewafer-like article does not need to be flat or change its flatnessduring a treatment (thermal elastic deformation) without additionalstress or degradation of holding reliability if the non-flatnessmagnitude is comparable with the gas gap size.

In processing applications and specifically for plasma processingapplications, there may be hcat input to the surface of the wafer-likearticle. Heat must be effectively removed so that there is not thermaldegradation of the wafer-like article (e.g., damage to semiconductor ICcircuits, thermally diffusing contaminants into a semiconductor wafer).Heat must be uniformly removed; otherwise, non-uniform processing forthermally dependent processes will occur (e.g., non uniform etching of amaterial from the wafer-like article with an etch process whose rate istemperature dependent). Consequently, the close proximity to the surfaceof the wafer-like article (i.e., less than 1 mm) provides efficient heattransfer across the wafer-like article to holder surface gap h. Also,because the holder's surface 11 has no large holes or discontinuities inits surface, only annular grooves which are small in size as compared tothe entire surface area of surface 11, the rate of heat transfer fromthe wafer-like article to the holder is substantially uniform across itsarea.

Referring to FIG. 5, flat wafer-like article facing surface 11 may beprovided with a plurality of annular grooves, such as the plurality ofannular grooves 112a-112d. As illustrated in FIG. 5, when a plurality ofannular grooves are employed, the quantity of annular grooves ispreferably chosen in multiples of two. Each annular groove 112a-112d hasa conduit 115a-115d connected to an opening 124a-124d in the ceilingsurface. The conduits are connected to a source of compressed gas. Eachof the conduits 115a-115d is selectively oriented in an oppositedirection from a conduit of an immediately adjacent annular groove. Forexample, in the embodiment shown in FIG. 5, two of the annular grooves112a and 112c, forming a first plurality of annular grooves, haveopenings 124a and 124c which are connected to a first plurality of gasconducting conduits, conduits 115a and 115c, which are oriented relativeto the annular grooves 112a and 112c to cause a counter clockwise flowaround each. The remaining two annular grooves 112b and 112d, forming asecond plurality of annular grooves, have openings 124b and 124dconnected to a second plurality of conduits, such as conduits 115b and115d, which are oriented relative to the annular grooves 112b and 112dto cause a clockwise flow around each. The opposite rotation of gas inthe adjacent annular grooves allows the rotational influences of thevortexes on the wafer-like article to cancel and the wafer-like article16 does not rotate relatively the flat wafer-like article facing surface11.

Referring to FIGS. 6 and 7, there is shown a fork-shaped end effector 40for picking up, transporting and holding wafer-like articles in anyorientation. With the fork-shaped end effector 40, the orientation ofthe wafer-like article may be changed during transport to allow, forexample, removal from a station in which it is face-down and loaded intoa wafer-like article holder, such as the holder described above, forprocessing with a face-up orientation. Also, the fork-shaped endeffector 40 can be made to allow removal from and into stations withlittle dimensional clearance. For examples where the wafer-like articleis a semi-conductor wafer or a glass plate used for lithographicexposure of semiconductor wafer patterns, standard packing is inmultiple, stacked configurations with little spacing between thearticles and only limited areas of open space around the article's edge.The fork-shaped end effector 40 can be made thin (e.g., less than about2.5 mm) to fit between stacked, wafer-like articles and with lateraldimensions that fit into open space around the edges.

The end effector 40 is formed by a substantially planar thin fork-shapedplatform 42 having first and second prong regions 43a-43b located atfirst end 44 and a base region 45 adjacent to second end 46. Theplatform further includes a pair of sides 48a and 48b. Preferably, thesides 48a and 48b taper outwardly from the first end 44 towards a point50 between the first and second ends. From point 50, the sides 48a and48b taper inwardly toward the second end 46.

An even numbered plurality of annular grooves 52a-52d are provided inthe platform 42. Each of the plurality of annular grooves is constructedin a similar fashion as described above. Each has inner and outer wallsand a ceiling surface which spans the inner and outer walls. The ceilingsurface of each of the plurality of annular grooves is provided with anopening. The openings in half of the even numbered plurality of annulargrooves, 52a & 52c, are supplied with a flow of gas by corresponding gasconducting conduits 54a & 54c, which are oriented relative to theannular grooves so as to direct the flow of gas in a counter clockwisefashion. The openings in the other half of the even numbered pluralityof annular grooves 52b & 52d are supplied with a flow of gas bycorresponding gas conducting conduits 54b & 54d, which are orientedrelative to the annular grooves so as to direct the flow of gas in aclockwise fashion. Annular grooves 52a and 52d are provided on first andsecond prong regions 43a and 43b respectively. Annular grooves 52b and52c are provided on the base region 45 of the platform 42. Gasconducting conduits 54a-54d may be made from hypodermic tubing.

The second end 46 of platform 42 is connected to extension member 56 andthe extension member 56 is attached to a gas manifold 58. The gasconducting conduits 54a-54d are connected to gas manifold 58 which hasinlet 60 connected to a supply of compressed gas (not shown).

Preferably, the platform is provided with first and second rim sections62a and 62b. The first rim 62a is positioned adjacent to the first end44, while second rim section 62b is positioned adjacent section to thesecond end 46. The rim sections prevent the wafer-like article frommoving laterally off of the platform.

Referring to FIG. 8, in the embodiment of the invention illustrated inFIG. 1A, the flat wafer-like article facing surface 11 is provided withrim 13 having an is inner surface 30, an outer surface 32 and an end 34.The rim is positioned along the facing surface's perimeter or edge. Therim 13 is intended to function as a boundary to retain the wafer-likearticle adjacent to the flat wafer-like article facing surface 11because the wafer-like article 16 has no mechanical contact and nofriction with the wafer-like article facing surface 11 and thewafer-like article 16 could shift relative to the facing surface 11.Preferably, the end 34 of the rim 13 extends out from the article facingsurface 11 by a distance L, which is typically more than 1 mm becausethe equilibrium gap size h is typically not more than 1 mm. While therim 13 may be continuous along the perimeter, alternatively, the rim 13may comprise a plurality of periodically spaced apart fingers, such asthat shown in FIG. 8. The fingers make the wafer-like article's edgesmore open to treatment agents than a continuous rim. In a preferredembodiment, the inner surface 30 of the rim 13, or the inner surface 30of each of the plurality of fingers comprising a rim 13, may be made tohave angle a which is greater than 90° relative to the plane formed bythe wafer-like article facing surface 11. The purpose of angling the rimis to reduce "shadow" effects caused by the rim 13 during treatment (asan example, plasma treatment) of the article 16.

According to the present invention, in any embodiment, such as thatshown in FIGS. 1A, 1B, 5, 6 and 7, for example, the flat wafer-likearticle facing surface 11 is provided with dimension D which is alwaysgreater than the dimension D₁ of the wafer-like article 16. In otherwords, the wafer-like articles which may be held by the apparatus of thepresent invention will always have dimensions which are less than thedimensions of the article facing surface 11. The wafer-like articlefacing surface 11 size D should be more than the wafer-like articledimension D₁ to allow the wafer-like article to take it's equilibriumposition without interference from the rim 13.

As will be understood from the foregoing description, according to thepresent invention, several embodiments of a wafer-like article holderhave been described which use gas dynamics to hold the article withoutphysical contact between the holder and the article. The presentinvention adequately provides a means for meeting the requirements forholding wafer-like articles for the purposes of processing the side ofthe wafer-like article facing away from the holder's surface,particularly with a reactive gas generated by an electrical dischargeand for transporting the wafer like article. It is to be understood thatthe embodiments described herein are merely illustrative of theprinciples of the invention. Various modifications may be made theretoby persons skilled in the art which will embody the principles of theinvention and fall within the spirit and scope thereof. Hence, thepresent invention is deemed limited only by the appended claims and thereasonable interpretation thereof.

What is claimed is:
 1. A holder for holding wafer-like articles (16)without physical contact between the holder and the wafer-like articlebeing held, the holder having a platform (10) with an article facingsurface (11), the holder characterized in thatthe article facing surfaceis provided with an annular groove (12) therein having depth (H), formedby an inner wall (17), an outer wall (18) and a ceiling surface (14)between the inner and outer walls, the ceiling surface further includingan opening (24) therein; and a gas conducting conduit (15), having adiameter (m) which is smaller than depth (H), is connected to theopening in the ceiling surface of the annular groove, wherein the gasconducting conduit is oriented relative to the opening in the ceilingsurface of the annular groove so as to cause a circular flow of gasaround the annular groove when a gas is flowed down the conduit andthrough the opening.
 2. The holder of claim 1, wherein the gasconducting conduit (15) is oriented so as to be substantially tangentialto the annular groove s circumference which is in the same plane as aplane formed by the article facing surface (11).
 3. The holder of claim2, wherein the ceiling surface (14) is substantially planar and the gasconducting conduit (15) forms an angle b with respect to the planeformed by the ceiling surface and wherein angle b is in the range of 0to 45 degrees.
 4. The holder of claim 1, wherein the ceiling surface(14) is substantially planar and the gas conducting conduit (15) formsan angle b with respect to the plane formed by the ceiling surface andwherein angle b is in the range of 0 to 45 degrees.
 5. The holder ofclaim 1, wherein the opening (24) in the ceiling surface (14) is smallerthan the ceiling surface.
 6. The holder of claim 1, wherein the ceilingsurface is provided with a plurality of openings (24a-24d), and whereinthe holder is further provided with a plurality of gas conductingconduits (15a-15d), each of the openings being connected to a gasconducting conduit, each ot the gas conducting conduits being orientedrelative to its respective opening so as to cause a circular flow of gasaround the annular groove in the same direction when a gas is floweddown the conduit and through the associated opening.
 7. The holder ofclaim 1, wherein the article facing surface (11) is provided with a rim(13) extending therefrom, the rim having an inner surface (30) whichfaces the annular groove, an outer surface (32) and an end (34).
 8. Theholder of claim 7, wherein the rim (13) is substantially continuous. 9.The holder of claim 7, wherein the rim (13) is formed from a pluralityof spaced apart fingers.
 10. The holder of claim 7, wherein the innersurface (30) of the rim (13) forms angle a with respect to the planeformed by the wafer-like article facing surface, and wherein angle a isgreater than 90 degrees.
 11. The holder of claim 1, wherein the gasconducting conduit (15) is formed from a hypodermic tube.
 12. The holderof claim 1, wherein the article facing surface (11) is further providedwith a second annular groove (112b) having depth (H) formed by a secondinner wall, a second outer wall and a second ceiling surface between thesecond inner and outer walls, the second ceiling surface furtherincluding an opening (124b) therein, anda second gas conducting conduit(115b), having a diameter (M) which is smaller than depth (H), connectedto the opening in the second ceiling surface of the second annulargroove, wherein the second gas conducting conduit is oriented relativeto the opening in the second ceiling surface of the second annulargroove so as to cause a circular flow of gas around the second annulargroove when a gas is flowed down the second conduit and through theopening, and wherein the circular flow of gas around the second annulargroove is in a direction which is opposite to the direction of thecircular flow of gas around the first annular groove.
 13. The holder ofclaim 12, wherein the platform further includes a rim (13) extendingfrom the article facing surface.
 14. The holder of claim 12, wherein theplatform (42) is fork-shaped so as to have a first prong region (43a), asecond prong region (43b) and a base region (45), andwherein the firstannular groove (52a) resides on the first prong region and the secondannular groove (52d) resides on the second prong region.
 15. The holderof claim
 14. wherein the first prong region (43a), the second prongregion (43b) and the base region (45) are provided with a rim (62a,62b).
 16. The holder of claim 1, wherein the article facing surface (42)is further provided with:a second annular groove (52b) having depth (H)formed by a second inner wall, a second outer wall and a second ceilingsurface between the second inner and outer walls, the second ceilingsurface further including an opening (124b) therein; a second gasconducting conduit (54b), having a diameter (M) which is smaller thandepth (H), connected to the opening in the second ceiling surface of thesecond annular groove, wherein the second gas conducting conduit isoriented relative to the opening in the second ceiling surface of thesecond annular groove so as to cause a circular flow of gas around thesecond annular groove when a gas is flowed down the second conduit andthrough the opening, and wherein the circular flow of gas around thesecond annular groove is in a direction which is opposite to thedirection of the circular flow of gas around the first annular groove; athird annular groove (52c) having depth (H) formed by a second innerwall, a second outer wall and a second ceiling surface between thesecond inner to and outer walls, the second ceiling surface furtherincluding an opening (124b) therein; a third gas conducting conduit(54c), having a diameter (M) which is smaller than depth (H), connectedto the opening in the second ceiling surface of the second annulargroove, wherein the second gas conducting conduit is oriented relativeto the opening in the second ceiling surface of the second annulargroove so as to cause a circular flow of gas around the second annulargroove when a gas is flowed down the second conduit and through theopening, and wherein the circular flow of gas around the second annulargroove is in a direction which is the same as the direction of thecircular flow of gas around the first annular groove; a fourth annulargroove (52d) having depth (H) formed by a second inner wall, a secondouter wall and a second ceiling surface between the second inner andouter walls, the second ceiling surface further including an opening(124b) therein; a fourth gas conducting conduit (54d), having a diameter(M) which is smaller than depth (H), connected to the opening in thesecond ceiling surface of the second annular groove, wherein the secondgas conducting conduit is oriented relative to the opening in the secondceiling surface of the second annular groove so as to cause a circularflow of gas around the second annular groove when a gas is flowed downthe second conduit and through the opening, and wherein the circularflow of gas around the second annular groove is in a direction which isopposite to the direction of the circular flow of gas around the firstannular groove: wherein the platform is fork-shaped so as to have afirst prong region (43a), a second prong region (43b) and a base region(45); and wherein the first annular groove (52a) resides on the firstprong region, the second annular groove (53d) resides on the secondprong region and the third and fourth annular grooves (53b, 53c) resideon the base region.
 17. The holder of claim 16, wherein the first prongregion (43a), the second prong region (43b) and the base region (45) areprovided with a rim (62a, 62b).